why do you want to know about functional programming?ats/talks/linq-ieee/linq-ieee.pdf · 04/23/07...

04/23/07 linq-

Why do you want to know about functional programming?

A look at LinQ in C# and (perhaps) Java

Axel T. SchreinerRIT — Computer Science

Eric Meijer

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http://www.cs.rit.edu/~ats/talks/linq-ieee/http://www.cs.rit.edu/~ats/cs-2006-1/14_linq.pdf

http://www.cs.rit.edu/~ats/talks/linq-ieee/

http://www.cs.rit.edu/~ats/talks/linq-ieee/

http://www.cs.rit.edu/~ats/cs-2006-1/14_linq.pdf

http://www.cs.rit.edu/~ats/cs-2006-1/14_linq.pdf

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Principles

Patterns

Examples

Implementation

Conclusions

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Principles

Map a query language to cascading method calls.

Implement the necessary methods for IEnumerable, XML- and database-access.

Sugar syntax liberally.

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What if...

Syntactic sugar patterned after SQL.

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struct Person { string first, last; int phone; }IContainer<Person> staff = ...Console.WriteLine( from x in staff where x.phone % 2 == 0 orderby x.last select new { x.first, x.phone });

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What if...

Syntactic sugar patterned after SQL.

Actions converted into messages,Expressions converted into functions.

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struct Person { string first, last; int phone; }IContainer<Person> staff = ...Console.WriteLine( from x in staff where x.phone % 2 == 0 orderby x.last select new { x.first, x.phone });

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What if...

Functional style [Eric Meijer]:

function as arguments,function composition for actions.

Anonymous class new { field ,... }.6

struct Person { string first, last; int phone; }IContainer<Person> staff = ...Console.WriteLine( staff .Where(x => x.phone % 2 == 0) .OrderBy(x => x.last) .Select(x => new { x.first, x.phone }));

➦

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Typing

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class C<T> { delegate R Func<R, T> (T arg); C<T> Where (Func<bool, T> predicate); C<T> OrderBy<K> (Func<K, T> keySelector); C Select (Func<U, T> selector);}

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Typing

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Typing

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Typing Pattern

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Query Expression Pattern

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from join... introduce iteration variable(s)

fromlet ...where

introduces iteration variable(s)introduces variablefilters

orderby rearranges sequence

selectgroup by

produces flat resultproduces structured result

into join...

produces iteration variable(s) to start over

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Query Class Pattern

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from join...

class C<T> { C<V> Join (...) C<V> GroupJoin (...)

fromlet ...where

C<T> Where (...)

orderby

O<T> OrderBy[Descending] (...) class O<T> : C<T> { O<T>ThenBy[Descending] (...) }

selectgroup by

C Select[Many] (...) C<G<Key,E>> GroupBy (...) class G<Key,E> : C<E> {...}

into join...

➦

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Exampleshttp://www.cs.rit.edu/~ats/cs-2006-1/code/adt/query/

http://www.cs.rit.edu/~ats/talks/linq-ieee/linq/

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http://www.cs.rit.edu/~ats/cs-2006-1/code/adt/query/

http://www.cs.rit.edu/~ats/cs-2006-1/code/adt/query/



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from let where ... / Where

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from x in xs where f

xs.Where(x => f)

retains only records satisfying a condition.

let x = e

defines name for value.

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delegate R Func<R,A> (A a);

IEnumerable<T> Where<T> (IEnumerable<T> container, Func<bool,T> predicate) { foreach (var c in container) if (predicate(c)) yield return c;}

Implementation

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public interface Func<R,A> { R f (A a); }public<T> Iterable<T> where (Iterable<T> container, Func<Boolean,T> predicate) { List<T> result = newList(); for (T c: container) if (predicate.f(c)) result.add(c); return result;}


IEnumerable<T> Where<T> (IEnumerable<T> container, Func<bool,T> predicate) { foreach (var c in container) if (predicate(c)) yield return c;}

Implementation

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Lazy Evaluation

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public<T> IterableImpl<T> where (final Func<Boolean,T> predicate) { return new IterableImpl<T>(this) { public Iterator<T> iterator () { return new IteratorImpl<T> () { final Iterator<T> c = delegate.iterator(); protected void getNext () { while (c.hasNext()) if (predicate.f(next = c.next())) { hasNext = true; return; } hasNext = false; eof = true;} };} };}

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int[] xs = { 1, 2, 3, 4, 5 };

from x in xs where x % 2 == 0 select x

Where(xs, x => x % 2 == 0)

Use

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List<Integer> xs = Arrays.asList(new Integer[]{ 1, 2, 3, 4, 5});where(xs, new Func<Boolean,Integer>() { public Boolean f (Integer x) { return x % 2 == 0; }})

int[] xs = { 1, 2, 3, 4, 5 };

from x in xs where x % 2 == 0 select x

Where(xs, x => x % 2 == 0)

Arrays are not Iterable.

Use

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Sugar: Extension Methods

A static class can define (static) extension methods.

If the class is imported, the methods can be called as instance methods:

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static class X { static R method (this A parm, ...) { ... }}

A a;a.method(...)

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Sugar: Lambda Syntax

An anonymous method definition can be significantly simplified.

Parameter and result types can be inferred.

Proposed for Java 7.

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parameter => expression(type parameter, ...) => { body }

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select / Select

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from x in xs select f

xs.Select(x => f)

creates new records.

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IEnumerable Select<U,T> (IEnumerable<T> container, Func<U,T> selector) { foreach (var c in container) yield return selector(c);}

Implementation

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public interface Func<R,A> { R f (A a); }public<U,T> Iterable select (Iterable<T> container, Func<U,T> selector) { List result = newList(); for (T c: container) result.add(selector.f(c)); return result;

}


IEnumerable Select<U,T> (IEnumerable<T> container, Func<U,T> selector) { foreach (var c in container) yield return selector(c);}

Implementation

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Lazy Evaluation

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public<U,T> IterableImpl select (final Func<U,T> selector) { return new IterableImpl(this) { public Iterator iterator () { return new IteratorImpl () { final Iterator<T> c = delegate.iterator(); protected void getNext () { if (hasNext = c.hasNext()) next = selector.f(c.next()); else eof = true;} };} };}

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select / Select[Many]

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from x in xs select f

xs.Select(x => f)

creates new records.

from x in xs from y in ys select f

xs.SelectMany(x => ys.Select(y => f))

creates new records — notice scoping.

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IEnumerable SelectMany<U,T> (IEnumerable<T> container, Func<IEnumerable,T> selector) { foreach (var c in container) foreach (var u in selector(c)) yield return u;}

Implementation➦

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public interface Func<R,A> { R f (A a); }public<U,T> Iterable selectMany (Iterable<T> container, Func<Iterable,T> selector) { List result = newList(); for (T c: container) for (U u: selector.f(c)) result.add(u); return result;

}


IEnumerable SelectMany<U,T> (IEnumerable<T> container, Func<IEnumerable,T> selector) { foreach (var c in container) foreach (var u in selector(c)) yield return u;}

Implementation➦

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int[] xs = { 1, 2, 3, 4, 5 };double[] ys = { 6.0, 7.0, 8.0, 9.0, 10.0 };

from x in xs where x > 3 from y in ys where y < 8.0 select new { y, x }

SelectMany(Where(xs, x => x > 3), x => Select(Where(ys, y => y < 8.0), y => new { y, x }))

Use

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Sugar: Tuple Types

A type can be created on the fly.

It has a read/write property for each field (and ToString).

The field types (and names) are inferred.

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new { name = expression, ... }

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int[] xs = { 1, 2, 3, 4, 5 };double[] ys = { 6.0, 7.0, 8.0, 9.0, 10.0 };

from x in xs where x > 3 from y in ys where y < 8.0 select new { y, x }

SelectMany(Where(xs, x => x > 3), x => Select(Where(ys, y => y < 8.0), y => new { y, x }))

Review: Use

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List<Integer> xs = Arrays.asList(new Integer[]{ 1, 2, 3, 4, 5});final List<Double> ys = Arrays.asList(new Double[]{ 6.0, 7.0, 8.0, 9.0, 10.0 });

selectMany( where(xs, new Func<Boolean,Integer>() { public Boolean f (Integer x) { return x > 3; } }), new Func<Iterable<HashMap<String,Object>>,Integer>() { public Iterable<HashMap<String,Object>> f (final Integer x) { return select( where(ys, new Func<Boolean,Double>() { public Boolean f (Double y) { return y < 8.0; } }), new Func<HashMap<String,Object>,Double>() { public HashMap<String,Object> f (Double y) { HashMap<String,Object> result = new HashMap<String,Object>(); result.put("y", y); result.put("x", x); return result; } }); } })

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orderby

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e.OrderBy(x => f) // .ThenBy(x => g) ...

from x in e orderby f [ascending|descending] // , g ,...

defines sort key[s] and direction.

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public<K extends Comparable<K>,T> Ordered<T> orderBy (Iterable<T> container, final Func<K,T> key) { // need a copied list for sorting List<T> values = newList(); for (T c: container) values.add(c);

return new Ordered<T>(values, new Comparator<T>() { public int compare (T a, T b) { return key.f(a).compareTo(key.f(b)); } } );}

Implementation:primary key

Comparator is composed from key

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public static class Ordered<T> extends Base<T> { protected final Comparator<T> order; protected boolean ordered; // true once sorted

public Ordered (List<T> values, Comparator<T> order) { super(values); this.order = order; }

public<K extends Comparable<K>> Ordered<T> thenBy (final Func<K,T> key) { // add to order return new Ordered<T>(super.getValues(), new Comparator<T>() { public int compare (T a, T b) { int c = order.compare(a, b); return c != 0 ? c : key.f(a).compareTo(key.f(b)); } } );}

next Comparator is composed from order and key

Implementation:secondary keys

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protected static class Base<T> extends AbstractList<T> { private final List<T> values; protected List<T> getValues () { return values; }

public Base (List<T> values) { this.values = values; }

public void add (int index, T element) { getValues().add(index, element); } // ...

protected List<T> getValues () { // use order List<T> values = super.getValues(); if (!ordered) { Collections.sort(values, order); ordered = true; } return values; }

Implementation:sort

get, remove, set, and size are also delegated; any use of values can be overridden:

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Use

sorts in dictionary order

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public static void main (String... _args) { List<String> args = Arrays.asList(_args); orderBy(args, new Func<String,String>() { // s => s.toLowerCase public String f (String s) { return s.toLowerCase(); }}) .thenBy(new Func<String,String>() { // s => s public String f (String s) { return s; } })

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joincomputes for pairs: inner record with same key as outer record

group-joincomputes for group of inner records with same key as outer record

group bygroups values computed from elements with equal key values

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etc.➦

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from join...

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from x in e join y in f on g equals h into i // ... z

e.GroupJoin(f, x => g, y => h, (x,i) => z)

defines anonymous iteration over groups.

from x in e join y in f on g equals h // ... z

e.Join(f, x => g, y => h, (x,y) => z)

defines anonymous iteration over pairs.

from x in e

defines iteration variable.

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Join

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delegate R F<R,A> (A a);delegate R F<R,A,B> (A a, B b);

C<V> Join<U,K,V,T> (C inner, F<K,T> outerKey, F<K,U> innerKey, F<V,T,U> selector) { var result = new C<V>(); foreach (var t in this) // from's container foreach (var i in inner) if (outerKey(t).Equals(innerKey(i))) result.Add(selector(t, i)); return result;}

selector combines records with matching keys.

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GroupJoin

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C<V> GroupJoin<U,K,V,T> (C inner, F<K,T> outerKey, F<K,U> innerKey, F<V,T,C> selector) { var result = new C<V>(); foreach (var t in this) { var group = new C(); foreach (var i in inner) if (outerKey(t).Equals(innerKey(i))) group.Add(i); result.Add(selector(t, group)); } return result;}

selector combines outer record with group of inner records and matching keys.

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group by

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from x in e group x by g

e.GroupBy(x => g)

groups elements by key value.

from x in e group f by g

e.GroupBy(x => g, x => f)

groups new values by key value.

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GroupBy

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C<Group<K,T>> GroupBy<K,T> (F<K,T> key) { return GroupBy(key, (F<T,T>)delegate (T t) { return t; });}

C<Group<K,E>> GroupBy<K,E,T> (F<K,T> key, F<E,T> selector) { var dictionary = new Dictionary<K,Group<K,E>>(); var result = new C<Group<K,E>>(); foreach (var t in this) { var k = key(t); Group<K,E> group; try { group = dictionary[k]; } catch { group = new Group<K,E>(k); result.Add(group); dictionary[k] = group; } group.Add(selector(t)); } return result; }}

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Group

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class Group<K,T> : C<T> { readonly K key;

public Group (K key) { this.key = key; }

public K Key { get { return key; } }}

Group preserves the common key value.

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Think functional!

A computer scientist invents this...

Action sequence: compositionCondition/selection: lambdas as argumentsNested selections: application of lambda Sorting: compose Comparator based on lambda arguments

No chance to see this in Java [56]...

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Think functional!


Action sequence: composition, lazy evaluationCondition/selection: lambdas as argumentsNested selections: application of lambda Sorting: compose Comparator based on lambda arguments


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Think functional!




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Think functional!




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Think functional!




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Think functional!


Action sequence: compositionCondition/selection: lambdas as argumentsNested selections: application of lambda Sorting: compose Comparator based on lambda arguments


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References

Erik Meijerhttp://research.microsoft.com/~emeijer/

C#: LinQhttp://msdn.microsoft.com/netframework/future/linq/

C# 3.0http://msdn2.microsoft.com/en-us/vcsharp/aa336745.aspx

Java 7: Closureshttp://www.javac.info/closures-v03.html

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http://msdn.microsoft.com/netframework/future/linq/

http://msdn.microsoft.com/netframework/future/linq/

http://msdn2.microsoft.com/en-us/vcsharp/aa336745.aspx

http://msdn2.microsoft.com/en-us/vcsharp/aa336745.aspx

http://www.javac.info/closures-v03.html

http://www.javac.info/closures-v03.html

why do you want to know about functional programming?ats/talks/linq-ieee/linq-ieee.pdf · 04/23/07...

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